1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof. The semiconductor device is equipped with MOS (Metal Oxide Semiconductor) transistors having different drain breakdown-voltages on a single SOI (Silicon On Insulator) substrate.
2. Related Art
In recent years, as weight savings and miniaturization of mobile electronic devices has been advanced, shrinkage of ICs for such electronic devices must be achieved. Especially for electronic devices equipped with liquid crystal displays, technologies for shrinking the area of IC chips for driving the displays by mounting low breakdown-voltage transistors for low voltage operations and high breakdown-voltage transistors for high voltage operations on single substrates (single chips) are strongly desired. Furthermore, in order to suppress costs, it is also desired to commonly use as much of the process for forming one of the low or high breakdown-voltage transistors in the process for forming the other.
However, high breakdown-voltage transistors sharing an element isolation process with low breakdown-voltage transistors need wider element isolation areas to ensure sufficient breakdown-voltages and further guard rings with high concentration diffusion layers or the like, which makes it difficult to reduce the areas of the transistor forming regions. Therefore, if the miniaturization of the low breakdown-voltage transistors is achieved, the areas of the high breakdown-voltage transistor forming regions cannot be reduced, thus preventing the areas of the drive IC chips from being dramatically reduced.
Furthermore, if the high breakdown-voltage transistors and the low breakdown-voltage transistors are formed on a single SOI substrate, the structures thereof need to be modified so as to form transistors suitable for the SOI substrate in order to avoid the floating body effect particular to SOI substrates, and accordingly, the design properties obtained from forming transistors on bulk silicon substrates cannot be utilized.
An object of the present invention is to provide a semiconductor device equipped with high breakdown-voltage transistors and low breakdown-voltage transistors on a single substrate and especially capable of reducing the areas of transistor regions without degrading the breakdown-voltage property thereby achieving miniaturization of the semiconductor device as a whole as well as avoiding effects particular to the SOI substrates such as a floating body effect in the low breakdown-voltage regions thereby utilizing conventional design properties, and is also to provide a method of manufacturing the above semiconductor device.